The present invention generally relates to a synchronous rotating apparatus for rotating a plurality of shafts, and more particularly to an apparatus for correctly synchronously rotating a plurality of shafts rotated at high speeds, for example, when applied to a positive displacement vacuum pump used in the manufacturing of semiconductors.
Taking a vacuum pump as an example of an application of the present invention, the drawbacks inherent in a conventional vacuum pump will be discussed hereinbelow.
A vacuum pump is necessary to produce a vacuum environment for a CVD device, a dry etching device, a sputtering device, an evaporation device, etc. in the manufacturing of semiconductors. Moreover, a strict standard is set with respect to the vacuum pump since the manufactoring of semiconductors is starting to require a cleaner and higher vacuum.
In the semiconductor plant, generally, a vacuum discharge system is constructed of a roughing pump (positive displacement vacuum pump) and a high vacuum pump (turbo molecular pump) to obtain a high vacuum. After a certain degree of vacuum pressure is attained from the atmospheric pressure by the roughing pump, the pump is switched to a high vacuum pump so as to thereby reach a predetermined high level of vacuum pressure.
FIG. 7 shows one screw-type vacuum pump which is a kind of conventional positive displacement vacuum pump (roughing pump), in which element 601 is a housing; element 602 is a first rotary shaft; element 603 is a second rotary shaft; elements 604 and 605 are cylindrical rotors supported by the respective rotary shafts 602 and 603; and elements 606 and 607 are grooves threaded in the outer peripheries of the respective rotors 604 and 605. In the conventional screw type vacuum pump, the first rotary shaft 602 and the second rotary shaft 603 are arranged so as to be parallel to each other within the housing 601, having rotors 604 and 605 thereon. The rotors 604 and 605 are provided with threaded grooves 606 and 607, respectively. When the recessed part (groove) of one rotor 606 or 607 is meshed with the projecting part (land) of the other rotor 607 or 606, a space is defined therebetween. As both rotors 604 and 605 are rotated, the volume of the space is changed so as to thereby suck and discharge air.
FIG. 8 illustrates one kind of conventional kinetic type vacuum pump (high vacuum pump), i.e., a vacuum pump of a screw groove type having a turbine blade. In the drawing element 801 is a housing; element 802 is a cylindrical rotor; element 803 is a turbine blade; element 804 is a screw groove; elements 805a and 805b are magnetic radial bearings which support a rotary shaft 807; and element 806 is a magnetic thrust bearing. The conventional vacuum pump with a turbine blade as shown in FIG. 8 has the rotor 802 inside the housing 801, and the turbine blade 803 and the screw groove 804 formed in the lateral upper and lower parts of the rotor 802. Each of the turbine blade 803 and the screw groove 804 impresses momentum to gas molecules, to execute sucking and discharging.
The conventional vacuum pumps and the vacuum discharge system in the combination of the conventional vacuum pumps described hereinabove specifically have the following drawbacks:
(a) Drawbacks of Roughing Pump (Positive Displacement Vacuum Pump)
The synchronous rotation of the two rotors 604 and 605 is achieved by timing gears 610a and 610b in the screw type vacuum pump of FIG. 7. That is, the rotation of a motor 608 is transmitted from a driving gear 609a to intermediate gear 609b further to one timing gear 610b of the rotor 605 which is meshed with a timing gear 610a of the rotor 604. The phase of the rotating angle of each rotor 604 and 605 is adjusted through the engagement of the timing gears 610a and 610b. Since the vacuum pump of this kind uses gears for the purpose of transmission of power from the motor and synchronous rotation of rotors, it is so designed that a lubricating oil 611 contained within a mechanical operating chamber where the gears are accommodated is supplied to the gears. At the same time, a mechanical seal 613 is provided between the mechanical operating chamber 611 and a fluid operating chamber 612 so as to prevent the lubricating oil from entering the chamber 612 where the rotors are housed.
In the above-described structure of the screw vacuum pump with two rotors, (1) many gears are needed for and transmission of power and synchronous rotation of the rotors, that is, a large number of components are used so as to thereby complicate the apparatus; (2) since the rotors are synchronously driven in a contacting manner using gears, the apparatus is not able to operate at high speeds and becomes bulky; (3) due to the abrasion, the mechanical seal must be regularly exchanged and the apparatus is not completely maintenance-free; and (4) the large sliding torque as a result of the mechanical seal brings about a great mechanical loss.
(b) Drawbacks of High Vacuum Pump (Kinetic Type Turbo Molecular Pump)
Similar to the roughing pump as depicted hereinabove, the turbo molecular pump is so constituted as to meet the requirement that the manufacturing environment of semiconductors should be clean. For instance, in the turbo molecular pump of a screw groove type having a turbine blade as shown in FIG. 8, magnetic bearings 805a, 805b, and 806 are employed in place of ball bearings which use oil lubrication. Therefore, the space where the bearings are accommodated is a vacuum in the turbo molecular pump. Although it is generally difficult to lubricate during the mechanical sliding motion a vacuum, the above use of magnetic bearings becomes a solution to this. Moreover, since an oil reservoir in the structure using ball bearings is not necessitated, the apparatus can be connected to a vacuum chamber in any position. Nevertheless, each shaft must be provided with an electromagnet, a sensor, and a controller, which disadvantageously results in a significant cost rise in comparison with the structure using ball bearings.
(c) Drawbacks of Vacuum Discharge System (a+b)
The conventional roughing pump (positive displacement vacuum pump) discharges air in the area of a viscous flow close to atmospheric pressure, and can only obtain a vacuum which is as low as about 10.sup.-1 Pa. On the other hand, the conventional high vacuum pump (turbo molecular pump) is workable up to approximately 10.sup.-8 Pa or so, but is unable to discharge in the area of a viscous flow close to atmospheric pressure. As such, in the conventional arrangement, the roughing pump (e.g., the earlier-mentioned screw pump) is first used to draw a vacuum approximately to 10.sup.0 -10.sup.-1 Pa, and subsequently the high vacuum pump (kinetic type turbo molecular pump) is used to attain a predetermined high vacuum.
In the meantime, with the recent complication in the manufacturing of semiconductors, a plurality of vacuum chambers have been independently driven, that is, a multi-chamber system has been a main stream for the manufacturing facilities. However, the above multi-chamber system requires a vacuum discharge system composed of a roughing pump and a high vacuum pump for every chamber, thus causing the system to be large-scale and complicated as a whole.
In order to solve the above (a), one of the inventors of the present invention has already proposed a positive displacement vacuum pump of a combination of a plurality of rotors in U.S. patent and application Ser. No. 738,902, wherein each shaft of the rotors is driven by an independent motor, and the rotors are synchronously rotated in a contactless manner. Accordingly, the vacuum pump is oil-free and miniaturized.
Further, in order to solve the above (b) and (c), the inventor has proposed a wide broad-band vacuum pump in the U.S. patent application Ser. No. 738,902 which is a composite pump having a kinetic type vacuum pump formed concentrically with one rotor of a positive displacement pump, thereby making it possible to draw from atmospheric pressure to a high vacuum by a single pump.
In the above-proposed arrangement, an incremental encoder is provided for each of a plurality of rotary shafts. A reference pulse is fed from the same pulse generator to each control circuit which controls the driving motor driving the corresponding rotary shaft so as to synchronously rotate the plurality of rotary shafts. Accordingly, the rotating speed and rotating phase of the driving motor, i.e., rotary shaft are controlled by the respective control circuit in accordance with the reference pulse.
Although the above proposal makes it possible to rotate a plurality of rotary shafts by the approximately same rotating frequency or rotating speed, it is difficult to correctly synchronize a plurality of rotary shafts within the absolute rotating angle (rotating position to the stationary coordinates).
A pair of rotors should be correctly rotated without contact each other while a fixed backlash is kept therebetween in the positive displacement pump. Even when the rotors are assembled and mounted correctly in position in accordance with signals from the respective incremental encoders, the rotors may shift during operation, partly because of the internal noise generated in the vacuum pump. For example, when the motors are started or stopped, a driving current of motors is suddenly changed, which causes an instantaneous increase in the switching noise. The other reason for the above shift is that an electromagnetic noise is generated from a plasma source when a vacuum pump is installed in the dry etching plant or a sputtering plant. These noises are superimposed on minute signals read from the encoders, thus inviting erroneous counting of the number of pulses indicative of the positional information of the motors. In the case where the incremental encoders have counting errors, the relative position of the two shafts is displaced and cannot be restored.
If an absolute encoder which can correctly detect the phase of a rotary shaft is employed, the phase shift of a plurality of rotary shafts can be detected through comparison, and the synchronization is achieved based the detection. However, many detecting slits must be provided in a rotating plate of the absolute encoder so as to correctly detect the absolute phase of the rotary shaft within 360.degree., and therefore, the outer diameter of the rotating plate is increased and the encoder eventually becomes scale. If the outer diameter of the rotating plate is increased, the rotating plate is easy to break or deform due to the centrifugal force when rotated at high speeds. As such, the synchronous method using the absolute encoder is often found to be difficult to be employed to correctly synchronize rotary shafts rotated at high speeds as in the vacuum pump described hereinabove.